An acousto-optic filter which employs the Bragg scattering mechanism generally operates as follows. A single-mode optical waveguide is formed in a substrate comprising an optical material such as LiNbO.sub.3. A surface acoustic wave (SAW) is generated on the substrate by a surface-acoustic-wave transducer. Multiwavelength light having a particular polarization state (TE or TM) enters the optical waveguide at one end. This light interacts with the surface acoustic wave generated by the surface-acoustic-wave transducer. The interaction of the multiwavelength light with the surface acoustic wave can result in a change in the polarization state of a particular one of the wavelengths in the multiwavelength light. Specifically, the propagation constant of a particular wavelength in the multiwavelength light combines (by addition or substraction) with the propagation constant of the surface acoustic wave to form the propagation constant of light with the particular wavelength, but with the orthogonal polarization state. When the propagation constants of the incoming light at a particular wavelength and the surface acoustic wave combine in this manner, efficient conversion of the polarization state of the incoming light with the particular wavelength takes place. Thus, in the filter input light, all of the wavelengths supposedly have the same polarization. In the filter output light, the polarization state of the particular wavelength has been changed, but the polarization state of all the other wavelengths remains unchanged. The optical wavelength whose polarization state is changed can be selected by selecting the propagation constant of the surface acoustic wave. The selected optical wavelength whose polarization state has been changed can be directed to one port, while the unselected wavelengths can be directed to a second band-reject port. Thus, an acoustic-optic filter of this type can be used to achieve a wavelength selective switch in a WDM optical communications system.
One problem with acousto-optic filters of the type described above is that the frequency response may have certain undesirable characteristics. In particular, prior-art acousto-optic filters have a frequency response with a passband which is highly peaked and which has significant side-lobes. The highly peaked passband means that there is little tolerance to an off-tuned input optical wavelength. The side lobes mean that there may not be sufficient separation between the selected wavelength and the unselected wavelengths for telecommunications system applications. Both of these problems cause crosstalk between channels.
The transmission bandpass characteristics of an acousto-optic filter of the type described above are determined by the acousto-optic interaction profile. The acousto-optic interaction profile is the amplitude profile of the surface undulation from the propagating acoustic wave along the path of interaction with the optical wave, i.e., along the optical waveguide. In other words, the acousto-optic interaction profile may be viewed as a modulation or envelope function which otherwise multiplies onto a monotonically sinusoidal acoustic wave on the surface. The acousto-optic interaction profile can be determined from a certain class of filter spectral response functions by using the so called inverse-scattering transform which resembles the well-known Fourier transform (see e.g. G. H. Song, et al "Inverse-scattering Problem For The coupled-wave Equations When The Refection Coefficient Is A Rational Function" Proc IEEE, 71, pp. 266-268, 1983; G. H. Song, et al "Design Of Corrugated Waveguide filters By The Gel'fand-Levitan-Marchenko Inverse-scattering Method", J. Opt. Soc. Am., A, 2, pp. 1905-1915, 1985). The filter frequency response can be determined from the acousto-optic interaction profile by solving the well-known coupled-mode equations.
FIG. 1 is a top view of a simple conventional acousto-optic tunable filter found in the prior art. The acousto-optic tunable filter 10 of FIG. 1 comprises a substrate 12 made of a piezoelectric acousto-optic material such as LiNbO.sub.3. A single-mode optical waveguide 14 is defined in the substrate 12. An acoustic waveguide 16 for guiding the surface acoustic wave is also defined in the substrate 12. The optical waveguide 14 is contained inside of the acoustic waveguide 16. The optical waveguide 14 is formed by diffusing Titanium into the substrate surface in the region of the optical waveguide. The acoustic waveguide is also formed by diffusing Titanium into the surface-acoustic-wave barrier regions 21 and 22. The dimension of an optical waveguide is much smaller than that of an acoustical waveguide so that the existence of the Ti-diffused optical waveguide in the middle of the acoustic waveguide does not prevent the acoustic waveguide from being a single-mode acoustic waveguide. A surface acoustic wave is launched in the acoustic waveguide 16 by applying an AC voltage to the interdigitated comb electrodes 23,24. The surface acoustic wave propagates in the acoustic waveguide 16 for a distance L and is absorbed by the surface-acoustic-wave absorber 25. Backward propagating surface acoustic waves are absorbed by the absorber 25'.
Multiwavelength light, wherein all the wavelengths supposedly have the same polarization state, enters the optical waveguide at the input end 26. The optical waves in the optical waveguide 14 interact with the surface acoustic wave in the acoustic waveguide in the acousto-optic interaction region of length L. As indicated previously, the propagation constant of the surface acoustic wave is chosen so that one of the optical wavelengths emerges from the output end 27 of the optical waveguide 14 with its polarization state altered, while the polarization state of the other optical wavelengths is unchanged.
The amplitude of the acoustic waves is constant for the entire interaction length L. Thus, the acousto-optic interaction profile, which is the amplitude envelope of the surface acoustic z wave is rectangular. The interaction profile is constant as a function of the z coordinate inside the region of length L and zero outside the region of length L. The interaction profile q(z) is plotted as a function of the normalized z coordinate in FIG. 2. The frequency response of the corresponding acousto-optic filter is plotted as a function of the normalized frequency in FIG. 3. (The center frequency of the frequency response in the physical world is equal to the nominal frequency of the selected wavelength whose polarization state is to be changed. However, in FIG. 3, this center frequency is normalized to zero). As can be seen in FIG. 3, the frequency response of the filter 10 of FIG. 1 has a highly peaked passband and the side-lobes are large.
FIG. 4 shows an acousto-optic interaction profile for a filter whose interaction profile is a half-period sinusoid. By solving the well-known coupled-mode equations, the frequency response of an acousto-optic filter with this interaction profile can be determined, and is plotted in FIG. 5. In comparison to the frequency response of FIG. 3, the frequency response of FIG. 5 has substantially reduced sidebands, but the passband is still highly peaked.
A prior-art filter structure which achieves the interaction profile of FIG. 4, and thus the frequency response of FIG. 5, is illustrated in FIG. 6. The filter structure 40 comprises the substrate 42, which is made of a piezo-electric acousto-optic material such as LiNbO.sub.3. An optical waveguide 44 is defined in the substrate 40. A pair of parallel acoustic waveguides 46, 48 are also defined in the substrate. The optical waveguide 44 is defined in the substrate by Titanium diffusion. The acoustic waveguides 46, 48 for guiding a surface acoustic wave are defined in the LiNbO.sub.3 substrate by Titanium diffusion in the surface acoustic wave barrier regions 50, 52, 54. Note that the optical waveguide 44 is located inside the acoustic waveguide 46. A second optical waveguide 45 may be formed inside the acoustical waveguide 48, so that the two acoustic waveguides are symmetric and efficient directional coupling between the two acoustic waveguides is achieved.
In the filter 40, multiwavelength light, wherein all of the wavelengths supposedly have the same polarization state, enters the optical waveguide 44 at the input end 55. The multiwavelength light leaves the optical waveguide 44 all the output end 56 with the polarization state of a selected wavelength changed and the polarization state of the other wavelengths unchanged.
The polarization state of the selected optical wavelength is changed by the interaction of the selected optical wavelength with a surface acoustic wave. The surface acoustic wave is launched by the interdigitated comb electrodes 57, 58 in the acoustical waveguide 48 and is absorbed by the absorber 59. Backwards propagating surface acoustic waves are absorbed by the absorber 59'. The surface acoustic wave couples from the acoustic waveguide 48 into the acoustic waveguide 46 as indicated by the dotted lines 60. The coupling is such that the surface acoustic wave in the acoustic waveguide 46 has an amplitude profile which is near zero at location 61, increases sinusoidally to a peak at location 62, and returns to zero at location 63 in accordance with the half-period sinusoid function. Thus, the acousto-optic interaction profile is as shown in FIG. 4 and the frequency response with a highly peaked passband is as shown in FIG. 5.
It is an object of the present invention to provide an acoustic-optic filter which overcomes the problems associated with the prior-art acousto-optic filters. Specifically, it is an object of the present invention to provide an acousto-optical filter whose frequency response has near-ideal bandpass filter characteristics, i.e., near-unity transmission inside the passband and near-zero transmission outside the passband.